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KGB bug
Early USSR radio bug · 1964

At the height of the Cold War, the Russian Intelligence Agency (KGB) used a variety of methods and equipment for eavesdropping on conversations in hotel rooms, embassies, private homes, etc. The device shown below was made around 1964 and was one of the first transistorized bugs.

The image on the right show the Russian KGB bug. The device measures 75 x 24 x 12 mm and is housed in a metal enclosure. It has three contact pins to which the power supply and the antenna are connected. The large black circle at the left is front of the crystal microphone.

The electronic circuit is extremely simple and consists of just one (Germanium) transistor, a simple coil and a couple of passive components. The tuned circuit consists of an adjustable 5 mm Ø coil with five windings, with a 30pF capacitive (crystal) microphone connected in parallel.
KGB bug compared to the size of a hand

As the bug is based on a free-running oscillator, it is rather unstable and sensitive to power variations. In order to reduce the so-called hand effect, the transmitter is built inside a metal enclosure. Furthermore, the frequency is adjusted by moving a (grounded) core towards the coil. As this core is grounded, a normal screwdriver can be used for the frequency adjustement at the side of the unit. The adjustment can be locked by means of another screw that is located at the top surface. Power is connected to the two-pin terminal at one of the short sides (right).

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When eavesdropping on a conversation, the capacity of the crystal microphone will vary slightly, causing variations in the resonance frequency of the tuned circuit, which effectively results in a Frequency Modulated (FM) signal. The KGB bug shown here was ideally suited for a wide variety of concealments, such as a piece of furniture (e.g. the leg of a table) or a decorative table piece. An external power source (e.g. a battery) was connected to the 2-pin terminal at the right, while a wire antenna was connected to the single terminal at the opposite side (above the microphone).

Due to the simple nature of this bug, it is easily detected and discovered. Considering the fact that the Russians also made more sophisticated – far less easy to find – bugs in the same era, it seems likely that this one was a bait that was sacrificed in the hope that the more sophisticated ones were not discovered. In most cases, more than one bug, each using a different operating principle, would be hidden at the target area, in the hope that at least one would not be found.

KGB radio bug of 1964 KGB bug of 1964 KGB bug compared to the size of a hand Antenna terminal Power terminals (plus and minus 6V DC) Frequency adjustment Microphone
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KGB radio bug of 1964
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KGB bug of 1964
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KGB bug compared to the size of a hand
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Antenna terminal
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Power terminals (plus and minus 6V DC)
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Frequency adjustment
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The KGB bug is housed in a molded brass enclosure that consists of two shells, held together by two recessed screws at the bottom. The two case-halfs are painted in grey hammerite on the out­side, and are silver-plated on the inside. Below is the device with the bottom shell removed.

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The interior consists of a printed circuit board (PCB), a tuned circuit and a crystal microphone. The PCB contains the passive components, whilst the only active part – the 2N499 transistor – is visible in the gap at the centre of the opposite side. At the far left is the crystal microphone that forms a tuned circuit with the coil that is fitted between the microphone and the PCB. To the left of the 2N499 transistor is a brass block with a screw mechanism that allows the transmission frequency to be adjusted by moving a brass core in and out of the coil, and locking it in place.

Interior of the KGB bug Microphone and tuned circuit Close-up of the internal circuit PCM component side PCB partly removed from the enclosure
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Interior of the KGB bug
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Microphone and tuned circuit
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Close-up of the internal circuit
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PCM component side
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PCB partly removed from the enclosure

Circuit diagram
Below is the circuit diagram of the 1964 KGB listening device featured here, as taken down from the actual device. The circuit is built around a single Philco 2N499 Germanium PNP transistor [1], which forms a straightforward oscillator in grounded-base configuration. The tuned circuit is at the bottom right, and consists of a coil (L1) (5 windings on 5 mm) and a crystal microphone with an internal capacity of 30pF. Power is applied via a diode at the top left to ensure correct polarity.

The antenna is connected at the bottom right, at the first winding of the coil from ground. A brass block allows a metal core to be inserted into the coil, in order to adjust the frequency in the range 110 to 120 MHz. The device in our collection is tuned at 116 MHz. Despite the fact that it is merely a simple free-running oscillator, the circuit is surprisingly stable and doesn't suffer too much from the so-called hand-effect. The device has a range of approx. 100 to 200 metres.

Audio in the bugged room is picked up by the crystal microphone at the bottom right, and is Frequency Modulated (FM) directly onto the tuned circuit, by the virtue of the fact that the micro­phone's capacity varies slightly with the sound. Tests have revealed that it is not very sensitive.

  • Type
  • Frequency
    110 - 120 MHz
  • Modulation
  • Microphone
  • Power
    4.5 to 6V DC
  • Dimensions
    76 × 24 × 11 mm (86 × 24 × 11 mm with terminals)
  • Weight
    54 grams
  • Range
    100 to 200 metres
  • Year
  1. 2N499 Germanium PNP transistor, datasheet
    Date unknown.
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Crypto Museum. Created: Friday 29 April 2016. Last changed: Tuesday, 11 August 2020 - 06:59 CET.
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